Orthogonal frequency division multiplexing (OFMD) is a technique where the channel is divided into a high number of parallel subcarriers which are transmitted simultaneously. Each subcarrier can be modulated by data, such as voice or text. The OFDM can be applied both in fixed and in wireless communications networks.
One of the most difficult problems in OFDM transmission is handling of large peak-to-average power ratios. A peak occurs in the signal power when data sent along different carriers align themselves in phase. In order to handle the peak in power, the power amplifier in the transmitter will provide gain for every peak. This is especially disadvantageous for instance in respect of the design of mobile terminals, because it is costly to provide terminals with power amplifiers with high power levels. Use of high power levels also shortens battery life. One solution to cope with this is that OFDM transmitters utilize clipping where the power peaks are clipped to a certain lower power level.
Clipping introduces distortion into the signal, and probability for bit-error decisions thus increases.